A terminology has been developed for chemicals that function as signals between organisms, paralleling the activity of the chemical cues involved in host-seeking behavior of parasitoids. All chemical attractants, arrestants, and so on that are important as modifiers of behavior between different organisms are grouped under the general term "semiochemicals." Pheromones are semiochemicals that serve to communicate between organisms of the same species; sex pheromones are an obvious example. Allelochemicals have effects between species and are further divided into those depending on whether the producing or receiving organism is helped or hurt by the signal. If the species producing the material is helped and the receiving one is harmed, the chemical is called an allomone. Examples of allomones include repellents that a stinkbug may produce to ward off predators such as ants or birds. A substance that harms the producing species but helps the receiving one is called a kairomone. The chemicals produced by insect hosts that serve as cues to parasitoids are kairomones because the parasitoid exploits them to the host's detriment. Usage is important here. A sex pheromone may attract a male to a female moth; but if a parasitoid cues on this chemical, the substance also functions as a kairomone. There are some materials that benefit both sender and receiver species. These are called synomones, and in the present context, the plant volatiles that attract parasitoids to host plants are synomones because they make it easier for the parasitoid to find herbivores damaging the plant.
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